P-815 mastocytoma cells can activate a suppressor cell in the spleen of allogeneic C57BL/6 mice. Suppressor cell activity is assyed in mixed lymphocyte culture. The suppressor cell is non glass-adherent XR- and cortisone resistant, and theta positive. The suppressor T-cell can be separated from the cytotoxic T-cell. Suppressor cells can also be activated by syngeneic tumors in a variety of tumor-mouse strain combinations. The splenic suppressor cells in these mice is also a non-adherent T-cell. Tumor cells, enclosed in cell impermeable diffusion chambers can also activate splenic suppressor cells.